i'm wondering how far can i stretch my Tagan 500W PSU.
is it going to be enough with an overclocked i5 CPU and say...a 7950 card?
according to this:
http://uk.hardware.info/productinfo/63256/tagan-superrock-500w#tab:specifications (i haven't opened my case in a while so i can't see it now)
it has a total of 40A on a split 12v rail so i think i could hold on to it for a future upgrade.
Actually, according
Tagan's page your PSU can only pull 360w (30A) max from the +12v rails.
If you can figure out the railing separation (or if a review had it laid out) and you can get your mobo/CPU on one rail and the video card on another, you should be fine. The problem with older power supplies, even a few years older, is that they lack amperage on the 12V rails. Over the last 20 years computers have shifted from using 3.3V and 5V rails to 12V rails. More modern power supplies with the latest specs put more and more emphasis and power on the 12V rail(s).
This said, if that 12V rail is split 20A/20A, you could get away with powering a 7950 easily. For reference, with my 7970 at a stock voltage overclock (typically what I game at) of 1125/1600MHz, my entire system pulls about 280-290W from the wall, or about 240W-250W actual after accounting for efficiency. However, if I crank the voltage to 1.3V on the core and get the clocks to 1.3GHz+, it'll pull 450W from the wall (or about 400W actual load). That's still below your 500W capabilities, but you might go over your 20A if you have anything else but the graphics card on a 12V rail. Therefore, if you don't overclock or lightly overclock, you'll be perfectly fine. However if you do push the clocks you might run into trouble.
So going by these readings, I'd say that Borealis's PSU is inadequate to safely run his GPU.
Aside from that, the ripple on that PSU is absolutely horrible.
I see, nothing inside the case except 2 HDDs so I think i'm good.
Lately, I get to recommend hardware to uninformed people quite a lot, and i'd hate to have them buy an uber VGA card just to have it borked by a PSU, so i'm trying to figure out a general rule on PSU requirements.
Dude, if I were you, I wouldn't risk it. If it were a higher quality power supply, like a SeaSonic or a PC Power & Cooling or a Corsair unit, then I'd say that you'd be fine because their single +12v rails supply a lot more amperage than the split rails. Not to mention that the ripple suppression is far superior to your Tagan. Honestly, I wouldn't push that Tagan anywhere close to its limits, it could very well fry and take everything else with it.
For peace of mind, get a GOOD power supply.
Power supplies are often the most skimped on and are probably the most important component of a computer. I learned the hard way years ago after switching from a cheapo Antec unit to a PC Power & Cooling. After that switch, I was able to get another 200MHz out of my CPU without any change in CPU VID because of the awesome ripple suppression and voltage regulation the PCP&C unit had.
Also, a good rule of thumb to follow is you want at least 10% overhead of total system draw. However, ~15-20% would be better for longevity as capacitor aging lowers efficiency and power output capacity.
Remember, a power supply does not supply it's rated power all the time, that is the number that it should be able to, theoretically, supply. So it's its
capacity to supply that power. However, a lot of lower end unit cannot supply even their advertised power without blowing capacitors out of them (literally).
That's actually more than 480's. 
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